Numerous recent studies have shown that acoustic metasurfaces (AMS) can be used to control reflected and transmitted wave fronts. Acoustic holograms created with traditional surfaces or AMS are created by searching a design space that considers the local amplitude and phase changes imparted to an incident wave front by the surface in order to construct the desired pressure field at one or more image planes. If the complete complex pressure field in an image plane is known, the angular spectrum can be used to efficiently determine the pressure magnitude and phase at a the AMS plane that is required to produce the desired image. The hologram design process typically requires iterative methods, such as the Iterative Angular Spectrum approach (IAS), to determine the amplitude and phase distribution on the hologram plane that is required to produce the desired pressure magnitude distribution in an image plane. Once the required properties at a hologram plane have been determined using this approach, AMS structures can be designed to generate the necessary local phase and amplitude conditions to a reflected or transmitted wave. One commonly applied approach to simplify the design or fabrication process is to consider phase-only modulation. While convenient, this approach limits the quality of the pressure field reconstruction that can be achieved, especially when attempting to create different holograms at different planes for a single-frequency incident wave. The control of magnitude and phase by an AMS is of significant interest because it enables one to completely define an entire reflected or transmitted wave field in a 3D region. In the present work, the physical limitations of simultaneously reconstructing multiple single-frequency holograms at different planes via phase-only metasurface designs are examined. The work considers the influence of the aperture size of pixels that form the metasurface, differences in quality between near and far field holograms, and image quality limits for hologram systems with multiple image planes. Methods for overcoming these limitations, including both amplitude and phase control by an acoustic metasurface, are also investigated and discussed. Practical considerations related to the implementation of a reflective acoustic hologram system both in simulation and experimentation are discussed. Specific considerations include AMS unit cell design and assembly, as well as the effect of overall error and design robustness on the reconstructed image quality based on known uncertainty in AMS construction and unit cell performance. Finally, results produced by the IAS are compared to both a full-wave simulation and a physical experiment demonstrating a multi-image acoustic hologram system produced by a reflective metasurface. Experimental methods for measuring reflective acoustic holograms are discussed, including design, production, and verification of the unit cells, construction of the metasurface, and measuring the pressure field at multiple image planes. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.


Physical Limitations of Phase-Only Metasurfaces for Acoustic Holograms

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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